114 THE RIDDLE OF MIGRATION 



Failure of the food supply has been perhaps most 

 frequently assumed to provide the stimulus. That 

 this cannot be the case with many species is patent 

 since they leave in July before the supply has at- 

 tained its peak. Food supplies are irregular, vary- 

 ing greatly in their dates of maturing and there are 

 years when they may be an almost complete failure. 

 It is extremely unlikely that they provide the stimu- 

 lus for a phenomenon that is outstandingly regular. 



Temperatures probably come next in popularity. 

 Our early migrants must again be excluded. As to 

 the later ones, we may get zero temperatures before 

 September is out or we may not get them until 

 January. Some years not even the smallest sloughs 

 may be frozen over by the beginning of November. 

 Periodically we get a killing frost in June or August 

 or on rare occasions even in July. Temperatures 

 can hardly be considered fundamental. 



Barometric pressure has been assumed to be the 

 stimulus by a number of writers. It has been 

 shown by various observers that high pressures may 

 precipitate both the northern and the southern 

 migrations (particularly the latter) but neither high 

 nor low pressures instigate migration during the 

 sedentary periods of summer and winter. We can 

 therefore consider pressure effects as incidental at 

 best. They may, and sometimes do, speed up 



